A wide variety of gas sensors are known. For example, a metal oxidant (SnO2) having semiconductor properties varies its resistance value when exposed to hydrogen gas due to reduction of the oxygen in the metal oxidant. Some semiconductor hydrogen sensors utilize the change in resistance value to determine gas concentration. A similar principle is used in hot-wire semiconductor hydrogen sensors, in which a metal-oxide semiconductor is fused to a platinum wire also serving as a heater and a bridge circuit detects a change in resistance value of a device. These semiconductor and hot-wire gas sensors use bulk materials that are detrimental to mass productivity and that require operating temperatures of equal to or higher than about 300° C.
Further, such electrochemical sensors are reported that have a solid electrolyte between a platinum electrode and a nickel electrode, and that measure electro motive force generated upon contact to hydrogen gas (see patent document 1).
Further, sensors using field-effect transistors are known to have high mass productivity and be operable at around room temperature. Non-patent document 1 reports a sensor using catalytic metal Pd (palladium) as a gate metal on an insulation film of a field-effect transistor.
Non-patent document 2 reports a hydrogen sensor using, similarly, Pt (platinum) as a catalytic metal.
Many conventional gas sensors have met, through long use, the problem of varied sensor output, i.e., long-term drift of sensor output. This makes it difficult to determine whether a change in sensor output is a response to gas or merely a result of drift. It is also difficult to determine whether a gas sensor is not responding because of breakdown or because, simply, there is no gas to be sensed. This necessitates periodic calibration of the gas sensor. In order to solve the problems, the inventors reported a field-effect transistor having a diagnostic function to determine whether a gas sensor is responding to gas; specifically, a proton-pumping gate FET that uses, as a gate structure, a three-layer structure of Pd, polyelectrolyte film, and Pt (see patent document 2).
[Patent Document 1] Japanese Patent Publication No. 4048444.
[Patent Document 2] Japanese Unexamined Patent Application Publication No. 2008-145128.
[Non-patent Document 1] J. Lundstrom, A. Spetz, U. Ackelid and Sundgren, “Catalytic Metals and Field-effect Devices—a Useful Combination.” Sensors and Actuators, B Vol. 1(1990), pp. 15-20.
[Non-patent Document 2] K. Tsukada, T. Kiwa, T. Yamaguchi, S. Migitaka, Y. Goto, K. Yokosawa, “A study of fast response characteristics for hydrogen sensing with platinum FET sensor.” Sensors and Actuators, B Vol. 114(2006), pp. 158-163.